Conventional engines maintain a fixed relationship between camshaft rotation and crankshaft rotation, thereby preserving the relationship between intake and exhaust valve events and piston motion. Alternatively, so called variable cam timing engines utilize a mechanism for adjusting this relationship for various advantages related to increased fuel economy and reduced regulated emissions.
To realize these benefits, accurate control of the cam timing is essential. One known method of controlling variable cam timing is now described. First, the actual cam timing is measured using a toothed wheel on the camshaft and a toothed wheel on the crankshaft. The time, or angle, between receiving pulses from the wheel on the crankshaft and the wheel on the camshaft represents the actual cam timing. Second, a desired cam timing is determined as a function of engine operating conditions. Third, an error signal is created from the difference of the desired cam timing and the actual cam timing. Fourth, control signals based on the error signal are sent to actuators capable of adjusting the cam timing. Such a system is disclosed in U.S. 5,363,817.
The inventors herein have recognized several disadvantages with the above system. First, the actual cam timing signal is susceptible to oscillations in the camshaft rotation. This results in unwanted high frequency measurement oscillations that cause the control system to respond in a less than optimal manner. Conventional filtering methods, such as low pass filters, can be applied to reduce these oscillations. However, these filters reduce the system response by adding a delay, causing slower performance. The size of the delay is proportional to the amount of filtering that is required. On actual operating engines, the necessary filtering to eliminate these oscillations causes an unacceptable reduction in system response.
A second disadvantage is that the actual cam timing signal contains further oscillations unless the manufacturing tolerance on the camshaft teeth location is maintained extremely tight. Requiring tight manufacturing tolerance increases system cost, which is unacceptable.